1. Field of the Invention
The present invention relates to a technique to stabilize gradation properties in an electrophotographic image forming apparatus.
2. Description of the Related Art
As an image printing system that is used in the image forming apparatus, such as a printer and a copying machine, the electrophotographic system and the ink jet system are known. The electrophotographic system is a system in which a latent image is formed on a photosensitive drum by using laser beams and development is performed by using charged color material (hereinafter, toner), and printing of an image is performed by transferring the developed toner image onto a transfer sheet and fixing the image. The ink jet system is a system in which ink is ejected from a plurality of eject ports by a heating body or a piezoelectric element and printing is performed by attaching ink to a printing medium such as paper.
In the case where, for example, the electrophotographic system is used for a printing unit, the electrophotographic processes, such as laser exposure, latent image formation on a photoreceptor, toner development, toner transfer onto a paper medium, and fixing by heat, are likely to be affected by the temperature and humidity around the apparatus, changes with the passage of time, etc. Because of this, the amount of toner that is ultimately fixed on a printing sheet changes each time printing is performed. It is known that such instability is not inherent only in the electrophotographic system, but occurs similarly in other various systems, such as the ink jet system and the thermal transfer system.
As a method of solving such trouble, there is a method in which a test pattern image is output from the image forming apparatus and the density thereof is measured, and then, a lookup table (LUT) is created from the results of the measurement and the characteristics of the printing unit are corrected.
In the correction using the LUT, in order to achieve the gradation properties that are aimed at, the maximum density, which is a target in the apparatus, is set in advance and correction is performed so that the gradation properties appear smoothly until the set maximum density is reached. However, depending on the condition of the apparatus, there is a case where a density that exceeds the set maximum density is produced, and therefore, in such a case, an excessive amount is suppressed by the LUT. At this time, due to the suppression of the upper limit value of the density, a problem of jaggies, discontinuity, etc., may occur in the portions, such as a character at the maximum density and a thin line. For this problem, a method has been proposed that solves the problem by causing a user to select the LUT for which the so-called terminal correction is performed to forcedly modify the upper limit value to the maximum value so as to guarantee that the input at the maximum density is output at the maximum density (see Japanese Patent Laid-Open No. 2001-094784).
On the other hand, for example, in the case where the maximum density that is targeted is 1.5, while an image at a density of 1.6 is output in a certain environment, the amount of color material exceeds the upper limit value as a result. In the case where the amount of color material per unit area exceeds a certain amount, which is the upper limit, as described above, another problem resulting from poor fixing of the color material may occur. Specifically, problems may occur, such as toner scattering in the case of the electrophotographic system, blurring of ink, mixing with another color, show-through at the time of printing of a plurality of pages, etc., in the case of the ink jet system. In particular, toner scattering may not only result in an image of poor quality, but also may result in damage to the apparatus main body. In order to avoid those problems, in the case where an image is formed, control is performed so that the total amount of color material per unit area is without fail lower than a fixed amount determined in advance. Then, for example, in the case of the electrophotographic color printer, a method is adopted in which the total amount per unit area is reduced by changing the combination of toner in four colors, i.e., cyan, magenta, yellow, and black while avoiding excessive restrictions. Specifically, it is common to adopt a method of suppressing the change in color tone and deterioration in the gradation properties to a minimum by increasing the amount of black instead of reducing the amount of cyan, magenta, and yellow by the same amount.
As described above, in the case where an output whose density exceeds the set maximum density is produced in order to reduce jaggies in the circumstances in which the limit value of the amount of color material per unit area is determined, there occurs another problem of poor fixing of the color material. In other words, jaggies and discontinuity in a line due to the suppression of the density upper limit value, and toner scattering and ink blurring due to an increase in the density by more than a supposed value are in the relationship of trade-off, but the method disclosed in Japanese Patent Laid-Open No. 2001-094784 does not take into consideration the balance between both of these concerns.
An object of the present invention is to appropriately control the tone level in accordance with the state of an image forming apparatus by taking into consideration the balance of the relationship of the trade-offs as described above.